Printed circuit board with multiple insulative magnetic layers

ABSTRACT

An inter-layer structure between a power layer and a ground layer of a printed circuit board includes an insulative magnetic structure with at least two layers of insulative magnetic substances with different complex magnetic permeability frequency characteristics.

BACKGROUND OF THE INVENTION

The present invention relates to a multilayer printed circuit board capable of preventing electromagnetic interference.

A high frequency current is applied to integrated circuits or large scale integrated circuits of the multilayer printed circuit board. The multilayer printed circuit board has a power layer, a ground layer and a signal layer. If the power layer and the ground layer form a large current loop circuits then a problem with the electromagnetic interference may be raised. In Japanese patent No 2734447, it is disclosed that in order to avoid this problem with the electromagnetic interference, a power de-coupling is emphasized, wherein the power layer comprises a wiring layer which includes an impedance-adding circuit. The impedance-adding circuit may be shaped in zigzag, crossing or spiral to avoid formation of the current loop. This power layer is sandwiched between underlying and overlying insulation layers which are made of an insulator mixed with a magnetic substance.

The above structure of the multilayer printed circuit board has the following problems.

The first problem is that when the integrated circuits or the large scale integrated circuits mounted on the multilayer printed circuit board are in operation, a high frequency current is applied to the power layer whereby the high frequency current applied to the power layer forms a large current loop which serves as a loop antenna which generates a radiation electromagnetic noise.

The second problem is that a current supplied from the power layer for switching the integrated circuits or the large scale integrated circuits causes variation in voltage of the power system comprising the power layer and the ground layer, whereby a stationary wave is caused and the power system serves as an antenna, so that the radiation electromagnetic noise with a frequency of the stationary wave is generated.

In Japanese laid-open patent publication No. 10-303563, a multilayer printed circuit board is disclosed. A single insulating layer is provided between the power layer and the ground layer. The single insulating layer include magnetic films which partially extend on selected regions of the insulating layer. The magnetic films are different in magnetic permeability, so that the insulating layer with the partially formed magnetic films serves as an electromagnetic shielding layer. Since the magnetic films are aligned in a single plane, it is possible for this conventional structure to attenuate the electromagnetic wave but only at a corresponding frequency to the frequency characteristic of the magnetic film. Accordingly, it is difficult for this conventional structure to attenuate the electromagnetic wave in a wide frequency range.

In Japanese laid-open patent publication No. 10-163636, it is also disclosed that magnetic sheet lamination structures are provided on both surfaces of a lamination of the power layer and the wiring layer. Each of the magnetic sheet lamination structures comprises a high magnetic permeability magnetic sheet and two low magnetic permeability magnetic sheets which sandwich the high magnetic permeability magnetic sheet. This magnetic sheet lamination structure is provided in order to increase the high frequency impedance with keeping the insulation between the power layer and the ground layer. The magnetic sheet lamination structure is not provided between the power layer and the ground layer which form a power system as a current loop. The magnetic sheet lamination structure is not provided in the current loop, for which reason the above structure having the magnetic sheet lamination structure is inapplicable to suppress the radiation electromagnetic noise.

Further, in Japanese laid-open patent publication No. 9-275251, various patterns of the conductive film applied to the multilayer printed circuit board are disclosed.

In Japanese laid-open patent publication No. 7-321463, a meshed interconnection layer applied for the multilayer printed circuit board are disclosed.

In Japanese laid-open patent publication No. 6-318650, plural stripe shaped power layers and ground layers applied for the multilayer printed circuit board are disclosed.

The foregoing conventional structures of the multilayer printed circuit boards are incapable of suppressing generation of a radiation electromagnetic noise and also incapable of attenuating the electromagnetic wave in the wide frequency range.

In the above circumstances, it had been required to develop a novel free from the above problem.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a novel multilayer printed circuit board free from the above problems.

It is a further object of the present invention to provide a novel multilayer printed circuit board capable of suppressing generation, in the wide frequency range, of a radiation electromagnetic noise from a power system comprising a power layer and a ground layer.

It is a still further object of the present invention to provide a novel multilayer printed circuit board capable of attenuating the electromagnetic wave in the wide frequency range.

It is yet a further object of the present invention to provide a novel multilayer printed circuit board capable of allowing a majority part of the high frequency current applied to the power layer to by-pass to a closest de-coupling capacitor so as to reduce, as many as possible, an effective high frequency current loop formed in the power system of the multilayer printed circuit board.

The present invention provides an inter-layer structure between a power layer and a ground layer. The inter-layer structure comprises an insulative magnetic layered structure which comprises at least two insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability.

The other present invention provides an inter-layer structure between a power layer and a ground layer. The inter-layer structure comprises a dielectric layered structure which comprises at least two dielectric substances which are different in frequency characteristic of complex dielectric constant.

The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a fragmentary cross sectional elevation view illustrative of a novel multilayer printed circuit board having an improved inter-layer structure provided between a power layer and a ground layer in a first embodiment in accordance with the first and second present inventions.

FIG. 2 is a fragmentary plan view illustrative of a power layer of a novel multilayer printed circuit board of FIG. 1.

FIG. 3 is a fragmentary cross sectional elevation view illustrative of the novel multilayer printed circuit board in use of FIG. 1

FIG. 4 is a fragmentary cross sectional elevation view illustrative of an inductor device of a power layer of FIG. 2.

FIG. 5A is a diagram illustrative of a frequency characteristic of a real part μ′ of a complex magnetic permeability μ*, wherein μ*=μ′−j μ″, of each of first and second insulative magnetic substances of first and second insulative magnetic layers laminated to form an inter-layer structure of a novel multilayer printed circuit board.

FIG. 5B is a diagram illustrative of a frequency characteristic of an imaginary part μ″ of a complex magnetic permeability μ*, wherein μ*=μ′−j μ″, of each of first and second insulative magnetic substances of first and second insulative magnetic layers laminated to form an inter-layer structure of a novel multilayer printed circuit board.

FIG. 6 is a fragmentary cross sectional elevation view illustrative of a novel multilayer printed circuit board having an improved inter-layer structure provided between a power layer and a ground layer in a second embodiment in accordance with the first and second present inventions.

DISCLOSURE OF THE INVENTION

The first present invention provides an inter-layer structure between a power layer and a ground layer. The inter-layer structure comprises an insulative magnetic layered structure which comprises at least two insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability.

It is important that the insulative magnetic layered structure comprises at least two insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability μ*, wherein μ*=μ−j μ″. The imaginary part μ″ of the complex magnetic permeability μ* is utilized to attenuate the electromagnetic wave in a wide frequency range. A frequency characteristic of a high impedance ;device depends o~n~a frequency characteristic of a real part μ′ of the complex magnetic permeability μ*, wherein μ*=μ′−j μ″. Further, if a stationary wave is present in the power system of the device, this stationary wave is attenuated by the imaginary part μ″ of the complex magnetic permeability μ* , whereby the electromagnetic noise radiated from the power system may be suppressed. The attenuation characteristic of the electromagnetic wave generated from the power system has such a spectrum shape as averaging imaginary parts μ″ of individuals of the at least two insulative magnetic substances. The at least two insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability μ* suppress the radiation electromagnetic noises from the power system in the wide frequency range

It is preferable that each of the at least two insulative magnetic substances comprises magnetic powders and a resin, and the at least two insulative magnetic substances are different in material of the magnetic powders.

It is also preferable that each of the at least two insulative magnetic substances comprises magnetic powders and a resin, and the at least two insulative magnetic substances are different in volume ratio of the magnetic powders.

It is also preferable that each of the at least two insulative magnetic substances comprises magnetic powders and a resin, and the at least two insulative magnetic substances are different in averaged particle diameter of the magnetic powders.

It is also preferable that each of the at least two insulative magnetic substances comprises a single kind of ferrite, and the at least two insulative magnetic substances are different in the kind of ferrite.

It is also preferable that each of the at least two insulative magnetic substances comprises plural kinds of ferrite, and the at least two insulative magnetic substances are the same in the plural kinds of ferrite and different in composition of ferrite.

It is also preferable that the insulative magnetic layered structure comprises laminations of at least two layers being respectively made of insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability.

The second present invention provides a multilayer printed circuit board having at least a signal layer, at least a power layer, at least a ground layer, and an inter-layer structure between the power layer and the ground layer, wherein the inter-layer structure comprises an insulative magnetic layered structure which comprises at least two insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability.

It is important that the insulative magnetic layered structure comprises at least two insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability μ*, wherein μ*=μ′−j μ″. The imaginary part μ″ of the complex magnetic permeability μ* is utilized to attenuate the electromagnetic wave in a wide frequency range. A frequency characteristic of a high impedance device depends upon a frequency characteristic of a real part μ′ of the complex magnetic permeability μ*, wherein μ*=μ′−j μ″. Further, if a stationary wave is present in the power system of the device, this stationary wave is attenuated by the imaginary part μ″ of the complex magnetic permeability μ* , whereby the electromagnetic noise radiated from the power system may be suppressed. The attenuation characteristic of the electromagnetic wave generated from the power system has such a spectrum shape as averaging imaginary parts μ″ of individuals of the at least two insulative magnetic substances. The at least two insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability μ* suppress the radiation electromagnetic noises from the power system in the wide frequency range.

It is preferable that each of the at least two insulative magnetic substances comprises magnetic powders and a resin, and the at least two insulative magnetic substances are different in material of the magnetic powders.

It is also preferable that each of the at least two insulative magnetic substances comprises magnetic powders and a resin, and the at least two insulative magnetic substances are different in volume ratio of the magnetic powders.

It is also preferable that each of the at least two insulative magnetic substances comprises magnetic powders and a resin, and the at least two insulative magnetic substances are different in averaged particle diameter of the magnetic powders.

It is also preferable that each of the at least two insulative magnetic substances comprises a single kind of ferrite, and the at least two insulative magnetic substances are different in the kind of ferrite.

It is also preferable that each of the at least two insulative magnetic substances comprises plural kinds of ferrite, and the at least two insulative magnetic substances are the same in the plural kinds of ferrite and different in composition of ferrite.

It is also preferable that the insulative magnetic layered structure comprises laminations of at least two layers being respectively made of insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability.

It is also preferable that the at least power layer comprises a wiring layer which has at least an inductor part serving as an inductor and the at least power layer is connected to a de-coupling capacitor

It is further preferable that the at least an inductor part comprises a plurality of divided lines and first and second branched parts which are connected to each other through the divided lines.

The third present invention provides an inter-layer structure between a power layer and a ground layer. The inter-layer structure comprises a dielectric layered structure which comprises at least two dielectric substances which are different in frequency characteristic of complex dielectric constant.

It is important that the dielectric layered structure comprises at least two dielectric substances which are different in frequency characteristic of complex dielectric constant ∈*, wherein ∈*=∈′−j ∈″. The dielectric losses of the at least two dielectric substances are utilized to attenuate the electromagnetic wave in a wide frequency range The attenuation characteristic of the electromagnetic wave generated from the power system depends upon a frequency characteristic of the dielectric losses of the at least two dielectric substances. The imaginary part ∈″ of the complex dielectric constant ∈* is utilized to attenuate the electromagnetic wave in a wide frequency range. A frequency characteristic of a high impedance device depends upon a frequency characteristic of a real part ∈′ of the complex dielectric constant ∈*, wherein ∈*=∈′−j ∈″. Further, it a stationary wave is present in the power system of the device, this stationary wave is attenuated by the imaginary part ∈″ of the complex dielectric constant ∈*, whereby the electromagnetic noise radiated from the power system may be suppressed. The attenuation characteristic of the electromagnetic wave generated from the power system has such a spectrum shape as averaging imaginary parts ∈″ of individuals of the at least two dielectric substances. The at least two dielectric substances which are different in frequency characteristic of complex dielectric constant ∈* suppress the radiation electromagnetic noises from the power system in the wide frequency range.

It is also preferable that the at least two dielectric substances are large in dielectric loss.

It is also preferable that the dielectric layered structure comprises laminations of at least two layers being respectively made of dielectric substances which are different in frequency characteristic of complex dielectric constant.

The fourth present invention provides a multilayer printed circuit board having at least a signal layer, at least a power layer, at least a ground layer, and an inter-layer structure between the power layer and the ground layer, wherein the inter-layer structure comprises a dielectric layered structure which comprises at least two dielectric substances which are a different in frequency characteristic of complex dielectric constant.

It is important that the dielectric layered structure comprises at least two dielectric substances which are different in frequency characteristic of complex dielectric constant ∈*, wherein ∈*=∈′−∈″. The dielectric losses of the at least two dielectric substances are utilized to attenuate the electromagnetic wave in a wide frequency range. The attenuation characteristic of the electromagnetic wave generated from the power system depends upon a frequency characteristic of the dielectric losses of the at least two dielectric substances. The imaginary part ∈″ of the complex dielectric constant ∈* is utilized to attenuate the electromagnetic wave in a wide frequency range. A frequency characteristic of a high impedance device depends upon a frequency characteristic of a real part ∈′ of the complex dielectric constant ∈*, wherein ∈*=∈′−j ∈″. Further, if a stationary wave is present in the power system of the device, this stationary wave is attenuated by the imaginary part ∈″ of the complex dielectric constant ∈*, whereby the electromagnetic noise radiated from the power system may be suppressed. The attenuation characteristic of the electromagnetic wave generated from the power system has such a spectrum shape as averaging imaginary parts ∈″ of individuals of the at least two dielectric substances. The at least two dielectric substances which are different in frequency characteristic of complex dielectric constant ∈* suppress the radiation electromagnetic noises from the power system in the wide frequency range.

It is preferable that the at least two dielectric substances are large in dielectric loss.

It is also preferable that the dielectric layered structure comprises laminations of at least two layers being respectively made of dielectric substances which are different in frequency characteristic of complex dielectric constant.

It is also preferable that the at least power layer comprises a wiring layer which has at least an inductor part serving as an inductor and the at least power layer is connected to a de-coupling capacitor.

It is also preferable that the at least an inductor part comprises a plurality of divided lines and first and second branched parts which are connected to each other through the divided lines.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a first embodiment of the novel multilayer printed circuit board has the following lamination structure. The novel multilayer printed circuit board 1 has first and second insulative magnetic layers 6 a and 6 b laminated to form an inter-layer structure which is sandwiched between a power layer 4 and a ground layer 3. The ground layer 3 is also isolated by a first insulating layer Sa from a first signal layer 2 a. The power layer 4 is also isolated by a second insulating layer 5 b from a second signal layer 2 b.

Namely, the inter-layer structure between the power layer 4 and the ground layer 3 comprises the first and second insulative magnetic layers 6 a and 6 b which respectively comprise first and second insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability μ, wherein μ*=μ′−j μ″.

It is preferable that each of the first and second insulative magnetic substances comprises magnetic powders and a resin, and the first and second insulative magnetic substances are different in material of the magnetic powders.

It is also preferable that each of the first and second insulative magnetic substances comprises magnetic powders and a resin, and the first and second insulative magnetic substances are different in volume ratio of the magnetic powders.

It is also preferable that each of the first and second insulative magnetic substances comprises magnetic powders and a resin, and the first and second insulative magnetic substances are different in averaged particle diameter of the magnetic powders.

It is also preferable that each of the first and second insulative magnetic substances comprises a single kind of ferrite, and the first and second insulative magnetic substances are different in the kind of ferrite.

It is also preferable that each of the first and second insulative magnetic substances comprises plural kinds of ferrite, and the first and second insulative magnetic substances are the same in the plural kinds of ferrite and different in composition of ferrite.

As shown in FIG. 2, it is also preferable that the power layer comprises a wiring layer which comprises a main wiring line part 8 and a plurality of branched wiring line parts 9 which have inductor parts 10 serving as inductors, wherein the inductor parts 10 are positioned near an IC or LSI.

As shown in FIG. 3, each of the inductor parts 10 may comprise a plurality of divided lines and first and second branched parts which are connected to each other through said divided lines.

As shown in FIG. 3, the first and second insulative magnetic layers 6 a and 6 b are laminated to form an inter-layer structure which is sandwiched between a power layer 4 and a ground layer 3. The ground layer 3 is also isolated by a first insulating layer 5 a from a first signal layer 2 a. The power layer 4 is also isolated by a second insulating layer 5 b from a second signal layer 2 b. The power layer 4 has inductor devices 10. The power layer 4 is connected through conductors 11 to LSIs 12 a and 12 b which are provided on the first signal layer 2 a. The ground layer 3 is connected through conductors 11 to LSIs 12 a and 12 b. The power layer 4 is connected through conductors 11 to de-coupling capacitors 13 and 13 b which are provided on the first signal layer 2 a. The ground layer 3 is also connected through conductors 11 to the de-coupling capacitors 13 a and 13 b. When the LSI 12 a is in operation, a high frequency current flows from the LSI 12 a through the conductor 11 to the power layer 4. This high frequency current further flows through the conductors 11 to not only the de-coupling capacitor 13 a which is positioned near the LSI 12 a but also other de-coupling capacitor 13 b which is positioned far from the LSI 12 a. The high frequency currents further flow from the de-coupling capacitors 13 a and 13 b through the conductors 11 to the ground layer 3. The current path through the de-coupling capacitor 13 b far from the LSI 12 a forms a large current loop. Further, in switching the LSIs 12 a and 12 b, a switching current is supplied from the power layer 4 to each of the LSIs 12 a and 12 b. This switching current supply causes variation in voltage of a power system which comprises the power layer 4 and the ground layer 3, whereby a stationary wave is present in the power system. The first and second insulative magnetic layers 6 a and 6 b are, however, provided which respectively comprise first and second first and second insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability μ*, wherein μ*=μ′−μ″. The imaginary part μ″ of the complex magnetic permeability μ* is utilized to attenuate the electromagnetic wave in a wide frequency range. A frequency characteristic of a high impedance device depends upon a frequency characteristic of a real part μ′ of the complex magnetic permeability μ*, wherein μ*=μ′−jμ″. Further, the stationary wave is attenuated by the imaginary part μ″ of the complex magnetic permeability μ*, whereby the electromagnetic noise radiated from the power system is suppressed. The attenuation characteristic of the +electromagnetic wave generated from the power system has such a spectrum shape as averaging imaginary parts μ″ of individuals of the first and second insulative magnetic substances. The first and second insulative magnetic substances which are different in frequency characteristic of complex magnetic permeability μ* suppress the radiation electromagnetic noises from the power system in the wide frequency range.

The above first and second insulative magnetic layers 6 a and 6 b provide high impedance to the high frequency current flowing through the power layer 4, for which reason a majority part of the high frequency current from the LSI 12 a flows through the de-coupling capacitor 13 a positioned near the LSI 12 a, whilst a majority part of the other high frequency current from the LSI 12 b flows through the de-coupling capacitor 13 b positioned near the LSI 12 b. No large loop is thus formed, whereby the radiation electromagnetic noise is suppressed. The frequency characteristic of the high impedance device depends upon the frequency characteristic of the real part μ′ of the complex magnetic permeability μ*, wherein μ*=μ−j μ″. If a stationary wave is present in the power system of the device in switching the LSIs, this stationary wave is attenuated by the imaginary part μ″ of the complex magnetic permeability μ* of each of the first and second insulative magnetic substances of the first and second insulative magnetic layers 6 a and 6 b, whereby the electromagnetic noise radiated from the power system may be suppressed. The attenuation characteristic of the electromagnetic wave generated from the power system has such a spectrum shape as averaging imaginary parts μ″ of individuals of the first and second insulative magnetic substances.

The following description will be focus on one available example of the first and second insulative magnetic layers 6 a and 6 b which comprise the same magnetic powders mixed in the same resin to have the same volume ratio of the magnetic powders but the first and second insulative magnetic layers 6 a and 6 b are different in averaged particle diameter of the magnetic powders. The first insulative magnetic layer 6 a comprises sendust powders and an epoxy resin, wherein a volume ratio of sendust powders to epoxy resin is about 64:36, and the averaged particle diameter of sendust powders is about 36 micrometers. The second insulative magnetic layer 6 a comprises sendust powders and an epoxy resin, wherein a volume ratio of sendust powders to epoxy resin is about 64:36, and the averaged particle diameter of sendust powders is about 8 micrometers.

FIG. 5A is a diagram illustrative of a frequency characteristic of a real part μ′ of a complex magnetic permeability μ*, wherein μ*=μ−j μ″, of each of first and second insulative magnetic substances 15 a and 15 b of the first and second insulative magnetic layers 6 a and 6 b, wherein the first insulative magnetic layer 6 a comprises sendust powders and an epoxy resin, where a volume ratio of sendust powders to epoxy resin is about 64:36, and the averaged particle diameter of sendust powders is about 36 micrometers, whilst the second insulative magnetic layer 6 a comprises sendust powders and an epoxy resin, wherein a volume ratio of sendust powders to epoxy resin is about 64:36, and the averaged particle diameter of sendust powders is about 8 micrometers. FIG. 58 is a diagram illustrative of a frequency characteristic of an imaginary part μ″ of a complex magnetic permeability μ*, wherein μ*=μ−j μ″, of each of first and second insulative magnetic substances 15 a and 15 b of the first and second insulative magnetic layers 6 a and 6 b, wherein the first insulative magnetic layer 6 a comprises sendust powders and an epoxy resin, where a volume ratio of sendust powders to epoxy resin is about 64:36, and the averaged particle diameter of sendust powders is about 36 micrometers, whilst the second insulative magnetic layer 6 a comprises sendust powders and an epoxy resin, wherein a volume ratio of sendust powders to epoxy resin is about 64:36, and the averaged particle diameter of sendust powders is about 8 micrometers. The attenuation characteristic of the electromagnetic wave generated from the power system has such a spectrum shape as averaging imaginary parts μ″ of individuals of the first and second insulative magnetic substances. The first and second insulative magnetic substances 15 a and 15 b different in frequency characteristic of complex magnetic permeability μ* can suppress the radiation electromagnetic noises from the power system in the wide frequency range.

A second embodiment according to the present invention will be described in detail with reference to the drawings.

With reference to FIG. 6, the novel multilayer printed circuit board has the following lamination structure. The novel multilayer printed circuit board 1 has first and second dielectric layers 16 a and 16 b laminated to form an inter-layer structure which is sandwiched between a power layer 4 and a ground layer 3. The ground layer 3 is also isolated by a first insulating layer 5 a from a first signal layer 2 a. The power layer 4 is also isolated by a second insulating layer 5 b from a second signal layer 2 b. The first and second dielectric layers 16 a and 16 b have large dielectric losses.

Namely, the inter-layer structure between the power layer 4 and the ground layer 3 comprises the first and second dielectric layers 16 a and 16 b which respectively comprise first and second dielectric substances which are different in frequency characteristic of complex dielectric constant ∈*, wherein ∈*=′−j ∈″. The dielectric losses of the first and second dielectric substances are utilized to attenuate the electromagnetic wave in a wide frequency range. The attenuation characteristic of the electromagnetic wave generated from the power system depends upon a frequency characteristic of the dielectric losses of the first and second dielectric substances. The imaginary part ∈″ of the complex dielectric constant ∈* is utilized to attenuate the electromagnetic wave in a wide frequency range. A frequency characteristic of a high impedance device depends upon a frequency characteristic of a real part ∈′ of the complex dielectric constant ∈*, wherein ∈*=∈−j ∈″. Further, if a stationary wave is present in the power system of the device, this stationary wave is attenuated by the imaginary part ∈″ of the complex dielectric constant ∈*, whereby the electromagnetic noise radiated from the power system may be suppressed. The attenuation characteristic of the electromagnetic wave generated from the power system has such a spectrum shape as averaging imaginary parts ∈″ of individuals of the first and second dielectric substances. The first and second dielectric substances which are different in frequency characteristic of complex dielectric constant ∈* suppress the radiation electromagnetic noises from the power system in the wide frequency range.

The above first and second dielectric layers 16 a and 16 b provide high impedance to the high frequency current flowing through the power layer 4, for which reason a majority part of the high frequency current from a first LSI not illustrated flows through a first de-coupling capacitor not illustrated and positioned near the first LSI, whilst a majority part of the other high frequency current from a second LSI not illustrated flows through a second de-coupling capacitor positioned near the second LSI. No large loop is thus formed, whereby the radiation electromagnetic noise is suppressed. The frequency characteristic of the high impedance device depends upon the frequency characteristic of the real pant μ′ of the complex dielectric constant ∈*, wherein ∈*=∈′−j ∈″. If a stationary wave is present in the power system of the device in switching the LSIs, this stationary wave is attenuated by the imaginary part ∈″ of the complex dielectric constant ∈* of each of the first and second dielectric substances of the first and second dielectric layers 16 a and 16 b, whereby the electromagnetic noise radiated from the power system may be suppressed. The attenuation characteristic of the electromagnetic wave generated from the power system has such a spectrum shape as averaging imaginary parts a ∈″ of individuals of the first and second dielectric substances.

Whereas modifications of the present invention will be apparent to a person having ordinary skill in the art, to which the invention pertains, it is to be understood that embodiments as shown and described by way of illustrations are by no means intended to be considered in a limiting sense. Accordingly, it is to be intended to cover by claims all modifications which fall within the spirit and scope of the present invention. 

What is claimed is:
 1. A structure comprising a power layer and a ground layer that sandwich an insulative structure that includes at least two insulative magnetic layers, each of said at least two insulative magnetic layers having a different complex magnetic permeability frequency characteristic.
 2. The structure of claim 1, wherein said at least two insulative magnetic layers directly contact each other and wherein said power layer and said ground layer each directly contact said insulative structure.
 3. The structure of claim 1, wherein each of said at least two insulative magnetic layers comprises a different type of magnetic powder.
 4. The structure of claim 1, wherein each of said at least two insulative magnetic layers comprises the same type of magnetic powder and a resin and wherein said at least two insulative magnetic layers have different volume ratios of said magnetic powder.
 5. The structure of claim 1, wherein each of said at least two insulative magnetic layers comprises the same type of magnetic powder and wherein an average particle diameter of said magnetic powder is different for said at least two insulative magnetic layers.
 6. The structure of claim 1, wherein each of said at least two insulative magnetic layers comprises only one type of ferrite and wherein the type of ferrite is different in each of said at least two insulative magnetic layers.
 7. The structure of claim 1, wherein each of said at least two insulative magnetic layers comprises plural ferrites of the same type, and wherein each of said at least two insulative magnetic layers has a different compositional ratio of said plural ferrites.
 8. A printed circuit board comprising: a signal layer; a power layer; a ground layer; and an insulative structure between said power layer and said ground layer that includes at least two insulative magnetic layers, each of said at least two insulative magnetic layers having a different complex magnetic permeability frequency characteristic.
 9. The printed circuit board of claim 8, wherein said at least two insulative magnetic layers directly contact each other and wherein said power layer and said ground layer each directly contact said insulative structure.
 10. The printed circuit board of claim 8, wherein each of said at least two insulative magnetic layers comprises a different type of magnetic powder.
 11. The printed circuit board of claim 8, wherein each of said at least two insulative magnetic layers comprises the same type of magnetic powder and a resin and wherein said at least two insulative magnetic layers have different volume ratios of said magnetic powder.
 12. The printed circuit board of claim 8, wherein each of said at least two insulative magnetic layers comprises the same type of magnetic powder and wherein an average particle diameter of said at least magnetic powder is different for said at least two insulative magnetic layers.
 13. The printed circuit board of claim 8, wherein each of said at least two insulative magnetic layers comprises only one type of ferrite and wherein the type of ferrite is different in each of said at least two insulative magnetic layers.
 14. The printed circuit board of claim 8, wherein each of said at least two insulative magnetic layers comprises plural ferrites of the same type, and wherein each of said at least two insulative magnetic layers has a different compositional ratio of said plural ferrites.
 15. The printed circuit board of claim 8, wherein said power layer comprises a wiring layer with an inductor part and further comprising a decoupling capacitor connected to said power layer.
 16. The printed circuit board of claim 15, wherein said inductor part comprises a plurality of branch parts that are connected to each other. 